What is a MAP sensor?

The MAP sensor is in charge of measuring the intake manifold vacuum pressure. On older systems, the MAP would take the place of the now known MAF (mass air flow) sensor. Newer vehicles often have both, a MAP and a MAF, where the MAP is relegated for EGR (a type of emission component) diagnostic and testing, and as a backup for the MAF. FOr the DIY and general consumer, testing these gadgets should not be too complicated. Get the scoop right here.

Theory of Operation

MAP sensors are three wire devices that measure intake manifold vacuum. In actuality, the MAP sensor measures the difference between intake manifold pressure/vacuum and atmospheric pressure. This is the reason why intake vacuum is NOT the same as MAP vacuum. Intake vacuum is atmospheric pressure minus MAP vacuum. With this in mind, the ECM makes the appropriate calculations as to the correct injector pulse. The MAP scanner PID is just MAP vacuum and should not be confused with intake vacuum. Few manufacturers do put out a manifold vacuum PID and Chrysler is one of them. In this case, the scanner PID for intake vacuum is a calculation (atmospheric press. minus MAP vacuum).

An engine’s vacuum is a good indicator of load. The MAP sensor outputs a DC voltage or frequency and its signal is inversely proportional, which means that as manifold vacuum increases voltage or frequency decreases. MAP sensors are also used as barometric (BARO) sensors. As soon as the ignition key is turned on, the ECM reads the MAP voltage or frequency signal and automatically takes that reading as atmospheric pressure. Some manufacturers have configured their ECM programming so that the barometric reading is updated during a WOT condition. Once the engine starts, the ECM uses the MAP, TPS and RPM signals as main inputs to calculate fuel control on MAP or SPEED DENSITY SYSTEMS (systems without a MAF sensor). The ECM modifies injector pulse-width according to the MAP signal output or engine load. This sensor is also used for ignition timing and on some systems is a backup for the MAF sensor. With dual MAP and MAF systems, the MAP sensor is primarily used to monitor the EGR valve operation.

MAP sensors are made of a piezoelectric material. This material is a form of crystal (Quartz) that when bent changes its internal resistance. MAP sensors output two different types of signals. Most output a voltage and usually work with a 5.00 Volt REF. The other type found mostly on FORDs, output a square wave at a certain frequency (FORD uses 159 Hz). As manifold vacuum increases the frequency output decreases.


MAP sensors are connected directly to manifold vacuum. This also means that any condition affecting the engine vacuum will also affect the MAP sensor reading. Conditions that affect engine vacuum are: EGR stuck open, clogged catalytic converter, engine mechanical problems, vacuum leak, ignition timing problems, valve timing adjustments and low fuel pressure. Also a shorted sensor feeding off the same sensor ground or 5.00 volt ref. line could cause a faulty MAP reading, due to the bad sensor shorting the MAP signal.

NOTE: It is important to understand that the MAP and RPM or Distributor Reference signals are the two most important inputs to the ECM for fuel control. These signals should be quickly assessed and analyzed when encountering any fuel control related problems.

NOTE: Beware of the fact that on a scan tool, some ECMs will substitute MAP reading on a rough running engine if it sees the signal out of range. Therefore the scan tool reading may be a substituted value that will throw the tech off during diagnostics.


All MAP sensors have three electrical wires going to it: sensor ground, reference voltage, and signal wire. The sensor ground is provided by the ECM for all the other sensors. A ground voltage drop test should be performed, between sensor ground and battery post ground to verify no more that a 100 mV voltage drop during KOER.

The reference voltage is also provided by the ECM and is a 5.00 volts regulated feed line. It provides the MAP sensor with its working voltage. A shorted 5 volt reference line, either the line wire or another sensor that is shorting it will directly affect the MAP sensor reading and therefore the entire engine.

The signal line is the signal return to the ECM. It provides the ECM with the actual MAP sensor reading. This is the line to tap to when performing actual tests and comparing them to the tables found here. A shorted MAP signal line will also adversely affect the sensor’s reading.

• The first step in analyzing a MAP sensor is the KOEO signal reading. If this reading is wrong to begin with, the ECM will react as if the vehicle is operating at higher altitude and the engine will run richer, since we need more fuel at high altitudes.

• Second, make sure that the scan reading is the same as the actual reading coming from the sensor itself. This will verify that the ECM is not substituting the signal values.

• Third, make a careful analysis using the “conditions that affect the MAP sensor” to find any causes that may be inducing a problem.

• Last, there is always the possibility that the ECM is at fault. However, a defective ECM will almost surely be in the sensor ground or 5 volt reference line and will probably affect other sensors. This sequential diagnostic analysis will lead you in the right direction.

By following these simple steps and carefully analyzing the corresponding wiring diagrams (s), it will surely point you in the right direction. If the reference voltage is gone, DO NOT assume that ECM is defective. If one of the sensors tied to the same reference line is shorted, it will bring down the reference line and all sensors connected to it. So, check your reference voltage.


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